The present invention relates to the field of drilling shot holes for seismic geophysical exploration in remote areas. More particularly, the present invention relates to a portable drill for creating shot holes through unconsolidated and hard rock geologic formations.
Certain geophysical seismic operations use explosive charges to generate shock waves for penetrating subsurface geologic formations. The shock waves are reflected from subsurface geologic structures and interfaces, and the reflected signals are detected with receivers or geophones located at the surface. In land-based geophysical seismic operations, shallow shot holes from five to several hundred feet are drilled into the upper geologic formations. Explosive charges are positioned within the shot holes, and the explosive charges are detonated to generate the shock waves.
Seismic operations are frequently performed in remote areas. The topography, ground cover, and inaccessibility of remote areas significantly restricts seismic equipment use, and these limitations are particularly significant in the creation of shot holes. Helicopters can position shot hole drilling equipment in remote locations, but conventional shot hole drilling equipment is significantly affected by the local soil conditions. Such soil conditions can include clays, shales, peat bogs, alluvial soils, granite, and other materials and combinations of materials.
A conventional rotary or reciprocating drill can generate a shot hole in unconsolidated soils such as topsoil and clay layers. Additionally, hard rock drills have been developed to generate shot holes in hard rock strata. However, conventional drilling equipment is slow and has difficulties in mixed geologic conditions where unconsolidated soil material overlays hard rock. In mixed geologic conditions, rotary and reciprocating drills can penetrate the unconsolidated soils but cannot penetrate the underlying hard rock at acceptable drilling rates. Hard rock drills are ineffective in unconsolidated soils because the drill mechanism is fouled by clay materials. Furthermore, hard rock drills using explosives are ineffective in unconsolidated soils because the explosives generate craters in the unconsolidated soils and create an unstable shot hole profile.
Alluvial soils present a different obstacle to shot hole drilling operations because alluvial soils often include rounded stones and boulders within a clay or other unconsolidated soil matrix. Even if a hard rock drill could adequately penetrate the unconsolidated soils, contact with an embedded hard stone can deflect the hard rock drill away from the desired path. This problem is particularly significant in shot hole drilling because the drill stem is relatively slender and is susceptible to deflection. If the shot hole is deflected off course by a hard stone in an alluvial soil, the accuracy of the explosive charge location will be affected and source position error can be introduced into the seismic processing calculations.
The elimination of source position error is particularly important in three dimensional seismic operations. A slight deviation of the shot hole due to deflection of the drilling mechanism will move the bottom hole shot location by several meters, depending on the offset and shot hole depth. This deviation can be significant in seismic data processing. Accordingly, a need exists for an improved shot hole drilling system capable of portable deployment and operation in complex, mixed geologic conditions with acceptable drilling rates.